Polyesters that are semicrystalline, particularly poly(butylene terphthalate) PBT, are used extensively in many applications that require good solvent resistance and good properties at elevated temperatures. They are ordinarily processed by injection molding, but there are many components of automobiles and other systems wherein such parts are hollow and to manufacture these by injection molding is very difficult and expensive. Many such parts can conceivably be made by blow molding provided the polymer system has adequate melt strength and melt viscosity and yields smooth high quality surfaces in the blow molded parts. Unfortunately, polyesters commercially available for injection molding and extrusion have melt viscosities which are too low to make them suitable for extrusion blow molding. It would be desirable to have blow moldable polyester compositions which provide moldings having smooth surfaces made from commercial injection moldable and extrusion grades of polyesters.
The addition of conventional di- and polyepoxides and, more recently, the addition of ethylene copolymers containing glycidyl groups have been suggested for increasing the melt strength and viscosity of polyesters, but are not suitable for blow molding large objects. The addition of catalytic cations to polyester compositions including ethylene copolymers containing glycidyl groups has also been suggested to provide greater increases in melt strength. Unfortunately, such catalyzed compositions can form small lumps when their heat history is severe as is the case in commercial molding operations where significant amounts of resin must be recycled as regrinds. These lumps cause surface imperfections in the molded parts which are objectionable in those application requiring a high degree of surface quality. Thus a need still exists for compositions which will yield large complex blow molded articles having smooth surfaces.